A filter is provided as an exhaust gas purification apparatus for an exhaust gas passage of an internal combustion engine in some cases. The filter traps the particulate matter (hereinafter referred to as “PM”) contained in the exhaust gas.
Patent Literature 1 discloses a system in which a pressure difference sensor (differential pressure sensor) is provided for outputting a signal corresponding to a difference between the exhaust gas pressure provided on the upstream side from a filter and the exhaust gas pressure provided on the downstream side from the filter (pressure difference between the front and the back of the filter). In Patent Literature 1, the pressure difference sensor is connected via a pressure introducing tube to the exhaust gas passage on the upstream side from the filter and the exhaust gas passage on the downstream side from the filter. Then, the PM accumulation amount accumulated on the filter is estimated by utilizing the output of the pressure difference sensor. Further, Patent Document 1 discloses a technique in which the state of the filter is detected by using the amount of change of the pressure in an intake manifold and the amount of change of the pressure difference between the front and the back of the filter.
Further, a filter has been also developed, in which a selective catalytic reduction NOx catalyst (hereinafter referred to as “SCR catalyst”) is carried thereon. When a reducing agent is supplied, the SCR catalyst selectively reduces NOx contained in the exhaust gas. The filter, which carries the SCR catalyst as described above, is hereinafter referred to as “SCRF”.
Patent Literature 2 discloses a system in which an aqueous urea solution is supplied via a nozzle from the upstream side to SCRF provided for an exhaust gas passage. In this case, urea is hydrolyzed, and thus ammonia is produced. Then, NOx is reduced by an SCR catalyst by using ammonia as a reducing agent. Further, in Patent Literature 2, a pressure difference sensor is provided for detecting the pressure difference between the front and the back of SCRF.